1. Field of the Invention
The present invention pertains to the field of electrical circuit assemblies, and more particularly to electrical circuit assemblies employing wires to electrically connect various circuit component terminals and/or conductors.
2. Background
Integrated circuits have many uses in industries ranging from communications to consumer electronics. A typical integrated circuit is formed by fabricating one or more semiconductor devices on a silicon wafer, often referred to as a chip. This chip is attached to a thermally conductive substrate, which may be suitable for mounting to a printed circuit board (PCB) or other carrier, as part of a greater electrical circuit assembly. A protective cover may be secured over the chip and substrate, wherein electrical contact leads extend through, or from under the lid, thereby forming an integrated circuit package. The package leads connect common terminals of the chip to other circuit elements located on the PCB.
Electrical connections between the leads of an integrated circuit package and contact terminals on the chip, as well as in-between contact terminals on the same or between chips, are often made with wires that xe2x80x9cjumpxe2x80x9d between the contact points, e.g., over an insulating structure or pathway. Such conducting wires are normally very small, typically having a diameter of a quarter of a millimeter or less. The wires are bonded to the contact terminals or leads using various techniques well known in the art, including thermosonic and ultrasonic bonding and soldering. Such wires are referred to herein as xe2x80x9cbond wires.xe2x80x9d
Electrical assemblies used in, for example, aerospace, automotive, and high frequency wireless communication applications can be subjected to intense vibrations. Due to their small diameters, bond wires are particularly susceptible to these vibrations at high frequencies. Over a sufficient number of cycles, the bond wires, including the bond pad where the bond wire connects to the lead or contact terminal, can fail due to fatigue fracture, which occurs when a material is deformed. The number of cycles until a material will fracture from fatigue increases as the deformation decreases.
One approach for preventing low-cycle fatigue failure due to thermal stresses involves encapsulating the wires with an epoxy, as described in U.S. Pat. Nos. 5,930,604 and 5,808,354, completing preventing movement of the wires and also preventing a short circuit between adjacent bond wires. The encapsulating material is poured over the assembly, including the bond wires, and solidified in order to immobilize the wires. This technique has several disadvantages. For example, the wires and respective bond pads are subject to thermal stresses due to differences in the thermal expansion coefficients of the encapsulating material and the materials that form the chip. Also, methods of manufacturing such chips can create voids, or gas pockets within the material, which create stress concentrations and lead to premature failure.
The present invention is directed to decreasing bond wire fractures in electrical assemblies and integrated circuit packages due to high frequency vibrations by decreasing the mechanical deformation of the bond wires as they respond to the vibrations, while still allowing for some movement of the wires to relieve thermal stresses and avoid other complications caused by the prior art technique of immobilizing the wires.
In one embodiment, terminals of an electrical component, such as, e.g., gate and drain terminals of a LDMOS power transistor, are connected by respective pluralities of bond wires to input and output leads extending from a substrate upon which the component is mounted. A damping material encompasses the bond wires, the damping material chosen to restrain but still allow movement of the bond wires in relation to the damping material. By way of non-limiting examples, the damping material may be various colloidal mixtures, perhaps but not necessarily liquid, or a viscoelastic material.
Other and further aspects and advantages of the invention will become apparent in view of the following detailed description of the preferred embodiments.